This application is related to U.S. patent application Ser. No. 451,396 filed concurrently herewith.
In the past, the transfer of data between remotely located terminal equipment such as PCs has been relatively slow and characterized by transmission errors. Telephone lines used for data transfers have one or more links which only carry analog signals. Typically, the local telephone line or link between a PC and the first switching station handles only analog signals. In order to serially transmit data from a PC over an analog phone line, a device such as a modem converts digital data to an analog signal. Although many long distant calls are converted and transmitted digitally, there still remains a number of analog links for most transmissions. The problem with analog transmission is that amplifiers used to boost the analog signals can distort the original signal causing error rates in the data roughly equivalent to the distance of the transmission. As a result of the relatively large number of errors possible from such transmissions, relatively small packets of data are transmitted at one time with frequent retransmission of the data as errors are detected. The result of such transmission errors is to slow down the data transfer rate with typical rates at about 1200 to 2400 bits per second.
In recent years, technology for digital data transmission has advanced. Digital transmission has a much lower error rate since amplifiers can easily and exactly restore a digital signal which only has two possible values. Digital transmission also permits multiplexing of various digitally encoded signals such as data, audio and video. Digital transmission is increasingly playing an important role in enhancing wide area computer networks which utilize telephone lines. One such digital network is referred to as Integrated Services Digital Network (ISDN). ISDN offers a wide range of services, most notably an increased transmission speed on the order of 64,000 bits per second.
In order to take full advantage of the potential of ISDN, data throughput as it relates to the speed at which data is transferred onto an ISDN line must be addressed. A typical PC has a terminal adaptor card for interfacing with a communications line. This card receives blocks of data from the PC, transmits the data over the communications line, and then sends an acknowledge signal to the PC. The time required to transfer a data block from the PC to the terminal adaptor card and to wait for an acknowledgment can be many times greater than the time required to transmit the data block over the ISDN line. Thus, the full potential offered by ISDN technology is not realized.